Energy Systems Lab FALL Experiment No DC Machines

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Kerrie Lawrence
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1 Objectives: Experiment No. 2-1 DC Machines The objectives of this experiment are to investigate the operation of dc generator under load, determine the characteristic Kaφ, the resistances r A (total resistance r A= r a + r b, where r a is the armature resistance and r b is the total brush resistance), r F (field winding resistance), r b (brush resistance) and the brushdrop voltage V BD for a particular DC machine. Apparatus: 1. 2 test tables 2. DC motor-generator set (lime green or cow) Ω field rheostat A Kenwood DC power supply phase resistive load bank 6. 4 Multimeters (To measure armature voltage on the Starter box, Field current for the motor using insertion plug, motor terminal voltage, and generator armature voltage.) A DC Ammeters for generator armature A DC Ammeter for motor armature 2 - Ammeter insertion plugs 1 - insertion plug with multimeter connectors 8. 1 Tachometer 1/8

2 Procedure: 1.Setup (a) Record the nameplate data of the machines used. (b) Plug in the test table. (c) Separately Excited Generator: Connect one of the machines of the DC motor-generator set as a separately excited generator. Please reference Figure 1. (i) (ii) (iii) (iv) (v) Connect the shunt field of the generator to the output of the 5A Kenwood DC power supply. Be sure to plug in the power cord for the Kenwood. Connect the resistive load bank in parallel to the 120V connection. Connect the resistive load bank through the test table to the armature of the DC generator. Keep the switches on the left side of the table open. Connect one of the 0-50 A DC ammeters such that the armature current of the DC generator can be measured. Connect a multimeter such that the armature voltage of the DC generator can be measured. (Connect into the reversal switch on the test table. Be sure the selection knob is selecting the correct lines on the voltage test table.) Figure 1: Separately Excited DC Generator 2/8

3 (d) Shunt Excited Motor: Connect up the other machine of the DC motor-generator set as a shunt excited motor. Please refer Figure 3 ON THE NEXT PAGE. A good method to follow your connections in an orderly fashion is to color code the lines on the diagram and coordinate with the color of connectors you use in the lab. Always start from the machine side when making connections! (i) (ii) (iii) (iv) Connect a multimeter to the starter box such that the armature voltage of the DC motor can be measured. The field and armature current of the motor should be measured from the starter box. Alternatively, when connecting through the test table, the field current can be measured with a multimeter from the table back connection line. (Instructor will demonstrate. This is where you will need the insertion plug with the multimeter connectors) The armature current can be measured on line through the test table with the 0-75A DC Ammeter. Turn on the field circuit breaker located on the 100 Ω field rheostat. Connect the tachometer to the shaft of the MG set. Adjust the height of the tach so that the connecting line is level with the shaft. Note the direction of rotation arrow on the MG set, and connect the tachometer to the end that will not cause the connector to unscrew while the machine is operating. (v) (vi) Make sure the switches on the left side of the table are off and insure that the ammeter shorting switches are closed. Have the instructor check your connections. Figure 2: Shunt Connected DC motor. (Do not use this diagram for making connections!!) 3/8

4 Figure 3: Connection of starter box to motor 2. Measure the magnetization curve of the DC generator (a) Close the circuit breaker on the 120 VDC supply panel. (b) Turn on the switches on the left side of the table that are connected to the motor. (c) Set the field current to a maximum i.e. set rheostat to minimum. (d) Check for the rotation of motor by lightly raising the handle on the starter box. If the rotation is in the correction direction and the generator field current set at zero, start the motor by slowly rotating the handle of the starter box. Pay attention to the armature current on the starting box to make sure that it doesn t spike, and bring the handle all the way up. If the motor rotates backwards; i. stop the motor, ii. open the circuit breaker on the 120 VDC panel, iii. reverse the shunt field leads (F1 and F2), iv. close the circuit breaker on the 120 VDC panel, v. try restarting. (e) Adjust the 100 Ω field rheostat until the motor is rotating at the nameplate speed of the motor. Continuously adjust the field rheostat such that the machine stays at the nameplate speed during the load and K A φ tests. 4/8

5 (f) Record the generator armature voltage due to the residual flux. (g) Note: To connect the Kenwood safely, be sure the voltage knob is turned all the way down before you enable the output. (h) With no load on the generator and the motor rotating at rated speed, use the Kenwood DC supply to increase the field current of the generator from zero to a value so that V agen is 110% of rated voltage. Incrementally increase the field current in 6 to 8 steps. Keep the speed at rated. Measure the armature voltage, V agen, and the field current, I fgen. The generated emf equals the armature voltage with no generator armature current; i.e., E g = V agen WAIT FOR OTHER GROUP TO REACH THIS POINT THE NEXT SECTION NEEDS TO BE DONE SIMULTANEOUSLY. 3. Measure the magnetization curve at 3/4 of the rated speed. (a) Adjust the 100 Ω field rheostat until the motor is rotating at the lowest speed possible. (b) The laboratory instructor will adjust the supply voltage such that the speed is 3/4 of rated speed. (c) With no load on the generator and the motor rotating at 3/4 of rated speed increase the field current of the generator from zero to a value so that V agen is 80% of rated voltage. Increase the field current in 6 to 8 steps and approximately match your field current readings from part 2(h). Keep the speed at ¾ rated. Measure the armature voltage, V agen, and the field current, I fgen. The generated emf equals the armature voltage with no generator armature current; i.e., E g = V agen (d) Leave the motor rotating. 4. DC Generator Load Test (Rated Speed) (a) The laboratory instructor will return the supply voltage to 120 V DC. (b) Adjust the 100 Ω field rheostat until the motor is rotating at the nameplate speed (c) Adjust the generator field current until the generator terminal voltage equals 120 V DC. Keep the generator field current constant at this value during the load test. (d) Measure Ia (armature current), If (field current), V (terminal voltage) of the motor. (e) Keeping the field current constant, for no load, 1/4, 1/2, 3/4, full load on the generator, measure and record: I a,gen, I a,motor, V T,motor, V a,gen I fmotor. 5/8

6 Be sure to have a person monitoring each of the ammeters to not exceed any limits (either the 50A or 75A of the meters or the machines nameplate values, whichever is lower). Also, be sure to systematically change the load to avoid any large changes in current which may damage the equipment. (f) Decrease the load current to a zero. (g) Turn off the armature circuit switches on the left side of the generator test table. (h) Bring the speed to a minimum value with the rheostat that controls the motor field current. (i) Turn off the motor by pressing the stop button on the starting box. (h) Turn off the switches on the left side of the motor test table. 5. Measurement of r A, r F, r b, r ip and V BD (a) Make sure that the circuit breaker on the 120 VDC panel is open. (b) Disconnect all leads to the starting box, motor-generator set, and the resistive load bank. (c) Connect the armature of the motor through a test table and the 1-φ resistive load bank to the distribution box. Make sure that originally the resistance bank is open circuited. (d) Turn on the circuit breaker on the 120 VDC panel. (e) To protect the machine, do not do the following for a long time. (f) Take six readings with current ranging from zero to 110% of rated current. For each current level measure the current, I a the voltage across the armature, Vt (between A1 and A2), the voltage across a brush, V BD /2=V b1 or V b2 (between the brush pigtails and a commutator bar under the brush), and the voltage between commutator bars under the two consecutive brushes V C. Use the handheld multimeter to measure the voltages. (g) Open the circuit breaker on the 120 VDC panel. 120 V + _ Table Supply A1 A2 Line 1 Line 2 Load 6/8

7 Figure 4 :Circuit set up for Part 5 Vb1 Vt Vc ra Vb2 ra Figure 5: Diagram of armature circuit. 6. Measurement of r f (a) Use a multimeter to measure the DC resistance of the shunt field resistance, r f, of both the DC machines. 7/8

8 Report: 1. From the readings of Part 2 (a) Plot E g versus I fgen. (b) K a φ versus I fgen, where Kaφ = Eg/ω (c) Use MathCAD s cspline and interp functions to plot 40 points from a curve fit of K a φ on the same graph as K a φ. Note: Plot both E g and K a φ on the same graph, but with different scales. (d) Predict and comment on the magnetization curve for higher field currents. 2. From the readings of Part 3 (a) Plot E g versus I fgen. (b) K a φ versus I fgen, where Kaφ = Eg/ω (c) Use MathCAD s cspline and interp functions to plot 40 points from a curve fit of K a φ on the same graph as K a φ. Note: Plot both E g and K a φ on the same graph, but with different scales. (d) Predict and comment on the magnetization curve for higher field currents. 3. From the readings of Part 4 Plot V agen versus I agen, 4. From the readings of Part 5 (a) Plot V BD versus I a (b) Select reasonable values for r b and V BD (c) Plot ra versus Ia where: ra r A r b = (V a -V BD )/Ia 5. From the armature resistance value obtained from the experiment, calculate the starter resistance that would be required to be inserted in series with armature to limit the starting current to twice the rated value. 8/8

Experiment 3. The Direct Current Motor Part II OBJECTIVE. To locate the neutral brush position. To learn the basic motor wiring connections.

Experiment 3. The Direct Current Motor Part II OBJECTIVE. To locate the neutral brush position. To learn the basic motor wiring connections. Experiment 3 The Direct Current Motor Part II OBJECTIVE To locate the neutral brush position. To learn the basic motor wiring connections. To observe the operating characteristics of series and shunt connected